This invention relates to a shadow mask support frame for a colour cathode ray tube.
Colour cathode ray tubes have a metallic sheet with a multitude of holes or slots in it, referred to as a xe2x80x9cshadow maskxe2x80x9d, disposed between the electron gun and the display screen. This shadow mask is intended to obtain a very sharp image by ensuring that the impacts of the electron beams on the display screen are situated precisely on the photophores disposed on the display screen.
The shadow masks are supported by frames with a generally rectangular shape which hold them in position close to the display screen and, possibly, ensure that they are tensioned, in order to limit any deformations resulting from local heating caused by the electron beams.
According to a known technique, a shadow mask support frame has two side uprights consisting of metallic tubes or angle steels and two end uprights consisting of angle steels or tubes placed on the side uprights and assembled by welding to the contact points. Because of the construction mode and the welding technique, the tubes or angle steels must be relatively thick in order to obtain sufficient rigidity. The frames thus made up are adapted to tensioned shadow masks but have the drawback of being heavy and difficult to fabricate with good geometric precision.
According to another known technique, a frame for a shadow mask consists of angle steels produced from thin strips which are assembled by welding.
In a variant, the frame is produced by pressing a metallic sheet, generally rectangular, and having a central cutout, also rectangular. The frames thus produced have the advantage of being lightweight but have the drawback of not being very rigid and because of this not very well suited to supporting tensioned shadow masks.
In order to improve the rigidity of the lightweight frames, it has been proposed, in particular in patent specification FR-2 749 104, to fabricate such frames from two thin metallic strips pressed against each other and having vertical and horizontal stiffening ribs. These frames are both lightweight and rigid, which makes it possible to use them for tensioned shadow masks, but they have the drawback of sometimes being difficult to fabricate.
In French patent application No. 99 02129, a shadow mask support frame was proposed in which the end uprights and the side uprights of the frame are generally tubular in shape and constitute a practically continuous closed hollow body containing at least one plane closed line situated entirely inside the hollow body.
Preferably, the hollow body consists of one or more thin metallic membranes folded so as to form all or part of the hollow body and assembled for example by welding.
Such a shadow mask support frame which is lightweight, rigid and easy to fabricate is indeed adapted to the case of generally flat frames, that is to say frames on which the longitudinal axes of the uprights are substantially in the same plane, the longitudinal uprights being connected to the end uprights at the corners of the frame.
Other types of frame are known which have end uprights (or horizontal uprights) and side uprights (or vertical uprights) whose axes or horizontal directions are situated in offset parallel planes.
In such frames, the end uprights generally consist of angle steels having a first flat wall situated in a reference plane of the frame substantially parallel to the position of the shadow mask carried by the frame and a second wall perpendicular to the first wall, by means of which a compression force is exerted on the frame, during the welding of the perforated mask, so that the frame is tensioned, when the compression force on the end uprights of the frame is released.
The side uprights of the frame generally have a substantially rectilinear main part and two end parts providing the connection and junction with the end uprights.
Each of the end parts of the side uprights has a section for joining to the end upright which is generally perpendicular or orthogonal to the main branch of the corresponding side upright and which is fixed to an end upright, in an arrangement perpendicular to the upright. Generally, the joining portion at the end of the lateral uprights is placed in abutment against the external surface of the first wall of the end upright and welded in this position to the end upright.
The welding of the abutment parts of the side uprights on the end uprights is effected by a welding method such as the TIG or MIG method. Such methods require using angle steels or tubes which are very thick, because they release a large amount of energy which is liable to deform the structures and melt the angle steels or tubes, if the walls are too thin.
In addition, the side uprights of the frame are generally produced by folding a thick tube or a bar with a square or rectangular nominal section. At the time of bending, the tube or bar are greatly deformed in the bending area. Normally a deformation in the form of a xe2x80x9cbonexe2x80x9d or xe2x80x9ccaskxe2x80x9d is observed.
The result is in particular poor precision with regard to the dimensions of the side uprights in the folded state and consequently poor precision with regard to the dimensions and geometry of the frame.
The end uprights, which are slightly curved, are generally fabricated by the cold or hot bending of a thick bar in the form of an L. Obtaining precise shapes and dimensions for the end uprights requires giving great care to the operations of bending the thick bar.
When the end uprights and side uprights of the frame are assembled, the welding in abutment of the end portions of the side uprights does not make it possible to obtain a high-precision assembly.
The frames obtained must therefore be planed after assembly, so that the dimensional precision necessary for the manufacture of the shadow mask frames for cathode ray tubes is obtained.
Using shadow mask frames comprising uprights with walls which are thick and therefore heavy requires also fabricating screen envelopes for the cathode ray tube which are themselves thick and therefore heavy. The cost of a cathode ray tube is determined mostly by the quantity of glass used for manufacturing the screen and the cone of the cathode ray tube. When the screen envelope is thick, the cone of the tube is itself massive. Cathode ray tubes with heavy frames are therefore extremely expensive.
When a massive frame is used, this frame, which is suspended in the front face of the screen envelope, is liable to become detached, during shocks suffered by the tube, for example during transportation, or damage to the studs attaching the frame to the envelope may occur.
In order to compensate for the overall expansion of the massive frame in the tube, it is necessary to use bimetallic strips constituting compensation elements welded to the frame uprights. Because the frame is heavy, it is necessary to use compensation strips with a thickness of around 1 to 3 mm, which are heavy and expensive to manufacture. To be able to weld the strips to the frame, generally by resistance welding, and to avoid deformation of the uprights of the frame, it is necessary to use thick tubes or profiled sections, which makes the frame even heavier.
The weight of the frame also makes it necessary to use high-power attachment springs.
A massive heavy frame has a high thermal capacity and heats up slowly when the cathode ray tube is switched on. Because of this, the time needed to obtain good colour stability for the tube may be relatively long. In service, the temperature of the frame may reach 80 to 100xc2x0 C.
During several steps of manufacturing the tube, the frame and shadow mask assembly is subjected to high temperatures of around 500xc2x0 C.
The expansion of the heavy massive frame might tear the shadow mask if there were no compensation system to de-tension the shadow mask.
For this purpose, it is possible either to adjust the coefficients of expansion of the shadow mask and of the frame so that the shadow mask is de-tensioned at around 500xc2x0 C., or use a compensation bar which expands more than the frame, so that the frame flexes and de-tensions the shadow mask.
The second solution, in the case of a heavy massive frame, requires the use of a heavy massive compensation bar.
When a frame is designed for a tensioned mask, it is possible to make provision for connecting the side uprights to the end uprights of the frame, close to the ends of these uprights.
In this case, when a uniform pressure is exerted on the end uprights, these flex between the two side uprights, so that the deformation in the central part of the end uprights is greater than the deformation towards the ends of the uprights connected to the side uprights.
The deformation is therefore not at all homogeneous along the length of the end uprights.
When the compression force on the end uprights is released, the traction on the shadow mask varies considerably according to the length of the end upright. It may be difficult to obtain good surface evenness of the mask and an even tension.
It has been proposed, in order to obtain a more even distribution of the tensions along the end uprights of a frame for a tensioned mask, to move the points of connection of the side uprights to the end uprights, to a certain distance from the end of each of the end uprights, for example up to a quarter of the length of the end upright, with respect to the two ends of this upright.
It is thus possible to control the distribution of the stresses on the shadow mask and to obtain a distribution of the stresses making it possible to control the surface evenness of the mask and to modify the vibration modes of the mask, with the possibility of damping the vibration.
The necessity of producing shadow mask frames in a heavy massive form, in the case of frames having side uprights in a plane offset with respect to the end uprights, stems essentially from the type of welding carried out on the abutting end portions of the lateral uprights and because the compression stresses applied to the end upright producing a shearing of the welded junction zones between the lateral uprights and the end uprights, when the shadow mask is assembled, require a strong connection.
The purpose of the invention is therefore to propose a shadow mask support frame for a colour cathode ray tube, rectangular in shape overall, having two substantially rectilinear end uprights parallel to each other comprising at least one wall substantially perpendicular to a reference plane of the frame, one edge of which is intended to receive a shadow mask in an arrangement substantially parallel to the reference plane of the frame, and two lateral uprights, tubular in shape overall, each having a main part with a substantially rectilinear axis and two end parts each connected to a portion for joining to an end upright, in an arrangement orthogonal with respect to the reference plane of the frame and parallel to the substantially plane wall of the end upright, the lateral uprights having axes parallel to each other situated in a plane parallel to the reference plane of the frame, this shadow mask support frame allowing a design of the frame which is lightweight and not very massive, to very precise dimensions and geometry, whilst obtaining good properties of rigidity and mechanical strength of the frame.
For this purpose, each of the joining portions is in contact through a lateral face with an internal face of the substantially plane wall perpendicular to the reference plane of the frame, so that the end uprights of the frame are in abutment on the joining portions of the lateral uprights.
According to a preferred embodiment, the two end uprights each comprise at least a first substantially plane wall in the reference plane of the frame and a second wall constituting the plane wall substantially perpendicular to the reference plane and therefore to the first wall, having in common with the first wall an edge with a longitudinal direction of the end upright and the two lateral uprights comprising portions joining to the end uprights, in an orthogonal arrangement with respect to the main part of the lateral upright and perpendicular to the first wall of the end upright and main parts having axes parallel to each other situated in a plane parallel to the reference plane of the frame, in an arrangement offset with respect to the reference plane of the frame; in this case, each of the joining portions is fitted in an end upright through the first wall of the end upright and fixed against the internal face of the second wall perpendicular to the reference frame.
In a particular embodiment, the end parts or extensions of the lateral uprights constitute, in pairs, continuous uprights in the direction of the end uprights joining the ends of the main parts of the lateral uprights, in pairs in order to constitute a complete flat frame. Each of the continuous uprights of the flat frame parallel to an end upright comprises at least one joining portion. The end uprights in which the joining portions are fitted have faces defining a reference plane parallel to the flat frame of the lateral uprights and more or less offset in a direction orthogonal with respect to the flat frame.
Each of the continuous uprights parallel to an end upright can comprise two joining portions spaced apart from each other in the longitudinal direction of the continuous upright and connected together by a connecting part between the extensions of the lateral uprights in order to constitute the continuous upright. Each of the continuous uprights can comprise a single joining portion disposed in a middle part of the continuous upright common to the two extensions of the lateral uprights constituting the continuous upright.